Field Emission Light sources (FEL) are of interest as an alternative to LED technology. FEL technology is environmentally friendly, avoids the large blue portion of the visible spectrum as used in LEDs and can be far more energy efficient as compared to LEDs in the UVC region.
Field emission is a phenomenon which occurs when a very high electric field is applied to the surface of material. This field will give electrons enough energy such that the electrons are emitted from the material.
In prior art devices, a cathode is arranged in an evacuated chamber, having for example glass walls, wherein the chamber on its inside is coated with an electrically conductive anode layer. Furthermore, a light emitting layer is deposited on the anode. When a high enough potential difference is applied between the cathode and the anode thereby creating high enough electrical field strength, electrons are emitted from the cathode and accelerated towards the anode. As the electrons strike the light emitting layer, typically comprising a light powder, the light powder will emit photons. This process is referred to as cathodoluminescence.
One example of a light source applying field emission light source technology is disclosed in EP1709665. EP1709665 disclose a bulb shaped light source comprising a centrally arranged field emission cathode, further comprising an anode layer arranged on an inside surface of a glass bulb enclosing the field emission cathode. The disclosed field emission light source allows for omnidirectional emission of light, for example useful in relation to a retrofit light source implementation. In addition tube forms are also of interest. This requires relatively long cathodes to be manufactured. Other shapes are possible such as flat lamps.
Several different materials may be used to create the necessary nanostructures used in order to achieve the extremely high electrical fields needed to operate a field emission light source at reasonable applied voltages. Carbon Nano Tubes (CNT's) have been used extensively to demonstrate the technology. However CNTs may erode during operation and carbon is deposited onto the anode, and will degrade the performance. CVD based nano-diamond films are also used, and work well, but these require long processes and expensive equipment and are therefore expensive to make.
An alternative is to use zinc oxide (ZnO). This material shows very little degradation, is inexpensive, and nanostructures may be created in several ways.
ZnO has excellent electron emission properties, particularly from rod or wire nanostructures. A cheap and attractive fabrication technique is to simply oxidize zinc metal or a zinc-carrying alloy in an oxygen-carrying atmosphere at elevated temperatures.
In order for the light source to operate with a uniform emission of photons over a relatively large area, the light powder must be uniformly bombarded by electrons and thus the uniformity of the electron emitting nanostructure properties must be controlled over a relatively large cathode area. In addition, for the manufacturing of such cathodes the uniformity must also apply when manufacturing large amounts of cathodes at the same time, i.e. all cathodes must be relatively equal. These processes may take several hours in process time, so when manufacturing high volumes, the cathodes must be manufactured in large quantities at the same time (also referred to as “batch manufacturing”).
In order to achieve a commercially attractive product, the properties of nanostructured elements must therefore be well controlled over large areas and it must be possible to manufacture large numbers of cathodes at the same time. Such considerations of uniformity over large areas or lengths as well as uniformity in larger reaction chambers are not found in present literature.
The present invention describes a highly uniform and reproducible zinc oxidation process yielding long ZnO nanowires with excellent and stable electron emission properties over long periods of time.